In a laser marking machine, it is arranged that a printing pattern of a divided image is displayed on a liquid crystal display screen (in which a transparent scattering type liquid crystal is employed) which serves as a liquid crystal mask; a laser beam is then scanned over the liquid crystal display screen and a printing position is moved so that the printing pattern of the divided image displayed on the liquid crystal display screen is printed at a position corresponding to this divided image on an article on which the printing pattern is to be printed.
In order to shorten the marking time and improve the efficiency of the operation, what is called "white space skipping" control is performed such that, of the printing pattern on the liquid crystal screen, only printing portions are scanned, non-printing portions being skipped without scanning.
By means of such "white space skipping" control, the benefit is obtained that the area of the scanning field can be reduced so the marking time can be shortened to an extent corresponding to the unscanned area of the field.
However, portions which are irradiated by the laser beam and portions which are not irradiated by the laser beam are interspersed, creating temperature differences within the plane of the liquid crystal mask.
The relationship between the transmittance of the liquid crystal and the drive voltage depends on temperature. If the drive voltage is fixed, the transmittance of portions of higher temperature increases. The drive voltage is in fact constant in the plane of the liquid crystal mask.
Temperature irregularities in the plane of the liquid crystal mask therefore appear as marking irregularities, adversely affecting the quality of the marking.
Thus, if "white space skipping" control is performed, a temperature distribution is created within the liquid crystal mask plane, producing unevenness of marking. Also, if "white space skipping" control is not performed, although unevenness of marking is suppressed, there is no way to shorten the marking time.
With the foregoing in view, it is an object of the present invention to achieve simultaneously both shortening of the marking time and suppression of irregularities in marking.